CRT has a strange disease



#9

Hi,

I'm facing an unusual problem on my 9845's CRT : since a few months, dozens of little white spots developed along the sides of the screen. Clearly they're on the outside of the tube, but they also seem to be under the surface since I can't wipe them off by any chemical or mechanical means. Above it all I'm afraid the problem may get worse as time passes...
In advance thank you for your help !

Regards
Marc


#10

I don't have one, so this is obviously just a guess, but I wonder whether the CRT's glass has some sort of protective coating that's starting to separate.


#11

Hi James,

Thanks for your answer ! If such a protective coating is used, it is actually glass. That makes me wonder whether the whole tube is not built with some kind of glass sandwich (two layers or more ?). Another fact : I found other spots, apparently on the inner side ! They're dark, hard to locate (need intense lighting).
Your guess is probably the right one. Sigh ! I don't think that such a phenomenon is reversible...
What surprises me the most is that the computer has always been stored in a safe place (no temperature / moisture hazards). Maybe just a few batches of these CRTs are concerned ? Anyway it still works !
Informations are welcome ! Thank you...

Regards


#12

Many CRTs are built with a protective layer of glass bonded to the tube like a car windshield. This is to protect the user from an imploding tube if it shatters. You might have a problem with the bonding layer. It is usually a clear rubber/plastic compound.

#13

Marc- This sounds just like the problem I had a year or so back- Basically mould-like stuff in the layer of clear 'adhesive' between the antiglare screen on the tube front and the bonded faceplate on the tube itself. Mainly around the egdes of the screen area.
I had to separate them completely and clean all the adhesive off both surfaces- a task that needs to be done with considerable care but is not all that difficult.

Hardest part was extracting the CRT from the case in the first instance-I had to get a very long philips screwdriver to reach all the way in from the back of the case to the mounting screws on the two bottom CRT mounting brackets.
Then mount the CRT on a stable and solid wooden frame before doing any work on it. The tube front needs to project well forward of the corner frame mounts so you have clear access to the edges of the 'gap' all the way round.

When reasssembling I just used a very small amount of replacement adhesive-clear silicon rubber- in the corners ONLY of the antiglare screen- the resulting gap between the tube faceplate and the antiglare glass has not caused any lack of visibility etc. While dust etc can now get in that gap it should also be fairly easy to clean it out again.

I am considering maybe filling in the gap down both sides of the antiglare glass with silicon rubber as well. That will give a bit more strength to the assembly-and will leave a top to bottom air path that can still be used for cleaning etc as required.

Dave Brown
Christchurch, NZ


#14

Keep in mind that there is some hazard involved when working on a CRT.
It can be very messy if a CRT implodes. Handle carefully of course, and
don't neglect personal protection, at least safety glasses or goggles.
If you have any spectators while working on this, be sure that they're
protected as well.

Don't forget that you can get a nasty shock from the CRT's anode. Even
in cases where the shock may not be truly life-threatening, the surprise
could be enough to cause you to drop the CRT. I've personally found that
even CRTs that have been stored "on the shelf" for months still can have
a charge. I've found that even discharging the tube doesn't entirely
solve the problem; after a while, there may be enough of a charge
available to give me a shock.

Regards,
James


#15

Hi !

James, Dave, David, I'd like to thank you all for these valuable informations. It's good to know that a solution exists. Doesn't seem to be very easy, but sure, I'll try it some day ! First, I need two important things : spare time, and enough room to work it out safely (these dear old machines take some place...)

Best regards,
Marc

#16

Hello Dave Brown,

Did you ever get your HP9100 working?


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